1. Field of the Invention
Apparatuses consistent with the present invention relate to a radio frequency and a microwave rectenna, and more particularly to an electrically small antenna in combination with an electronic chip of a radio frequency identification (RFID) transponder.
2. Description of Related Art
An RFID transponder is a tag device that can respond to being read by sending a content of its embedded memory by backscatter communication to an interrogator, i.e. a reader. A passive RFID transponder has no battery; instead, it gets all the needed energy to send a signal to the reader, from a carrier signal of the reader.
Generally, the RFID transponder includes an application specific integrated circuit (ASIC) connected to an antenna. Low cost planar antennas for RFID transponders with substantially small electrical size have been heavily focused on in recent years. The reason being is that, currently, an antenna size of even a quarter of a wavelength is precluded from many applications.
Small antennas are constrained in their behaviour by a fundamental limit: the smaller the maximum dimension of the antenna, the higher the Quality Factor (Q), or equivalently, the narrower the frequency bandwidth. Accordingly, the art of antenna miniaturization is always an art of compromise between size, bandwidth, and gain. In the case of a planar antenna, a good compromise is usually obtained when most of the given areas of the antenna strongly participate in the radiation phenomenon.
The inventors have disclosed a small antenna that can operate over an enhanced bandwidth without affecting the radiation pattern, gain, and polarisation purity. Such antenna is described in Korea Patent Application No. 10-2005-0026496. The direct implementation of the antenna described in Korea Patent Application No. 10-2005-0026496 for an RFID transponder design faces important challenges. The ASIC included in the RFID transponder has essentially a complex input impedance with a substantial capacitive reactance. Therefore, the antenna impedance should be complex conjugate matched to the impedance of the ASIC. Impedance matching between a transponder ASIC and an antenna is critical for overall RFID system performance. Specifically, the mismatch very strongly affects a read range, i.e., a maximum operating distance between a reader and a transponder since the power radiated by the reader is rather limited due to certain safety regulations and other legislation. Also, a passive RFID transponder extracts its operating power by rectifying interrogation signals delivered by the antenna.
A rectifying circuit is within an ASIC and includes diodes, such as Schottky diodes, and capacitors, resulting in a complex input impedance with substantial capacitive reactance. Typically, the impedance of an ASIC comes to a few or tens of Ohms and a few hundred reactive (capacitive) Ohms. Thus, the ratio of the reactance to the resistance is very high.
Generally, the circuit including an antenna and a rectifying circuit is termed as a rectenna.
A related art rectenna is described in Korea Patent Application No. 10-2005-0026496.
FIG. 1 is a diagram illustrating a rectenna 1 in the related art. The rectenna 1 includes a rectifying circuit embedded into an ASIC 2 of a transponder and a connected antenna 3. The antenna 3 is shown separately in FIG. 2. The electrically small antenna 3 has a dielectric substrate 31; a thin metal layer 32 formed on a top surface of the substrate 31; and a slot pattern 33 within the metal layer 32. The metal layer 32 with the slot pattern 33 constitutes a radiating part of the antenna 3. The slot pattern 33 is shown separately in FIG. 3. The slot pattern 33 includes a main slot 331; four slot arms 332, 333, 334 and 335 terminating the main slot 331 at both ends of the main slot 331; a system of transverse slots 336 placed along the main slot 331 in such a manner that the system of the transverse slots 336 is divided into two mirror-symmetrical halves by the main slot 331. The antenna 3 is functionally fed by a direct inlet of an electronic ASIC chip in the slot pattern 33 at a feeding point 337.
Radar Cross Section (RCS) characterizes how an antenna scatters electromagnetic energy of an incident wave field. For backscatter communications, the RCS of a rectenna is a factor because a modulated RCS is essentially used for the transmission of data from a transponder to a reader.
Good impedance matching between an ASIC and an antenna increases the read range of an RFID system. The maximum read range also depends on the gain of the transponder antenna. Enhancement in gain basically allows an increased reading range.
The related art rectenna provides a conjugate matched small antenna having enhanced RCS for an overall increased bandwidth without affecting the radiation pattern and polarization purity. However the gain of the rectenna can be further enhanced.
Accordingly, it would be desirable to provide a rectenna with an electrically small conjugate matched antenna having an enhanced gain for an overall increased bandwidth without strongly affecting polarization and RCS characteristics.